A conventional aromatic saturated polyester has excellent physical properties as a thermoplastic resin, such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), or polyethylene-2,6-naphthalene dicarboxylate (PEN), and is used in a wide range of products as fibers, films, plastics, and the like. In the plastic field, a molded product thereof is widely employed for bottles, sheets, containers, daily commodities, automobiles, machinery parts, electronic and electric materials, building materials, various industrial goods, or the like.
In recent years, from the viewpoint of resource saving and environmental conservation, the need for recycling used plastic products collected from factory production processes or a general consumer market has been globally recognized. In particular, the recovery and recycling of used PET bottles, films, and the like is being actively promoted. Polyethylene terephthalate contains, in addition to terephthalic acid, a small amount of isophthalic acid. Further, a polyester containing cyclohexane dimethanol such as PETG is also mixed in polyethylene terephthalate in recent years. Therefore, the polyethylene terephthalate herein is collectively referred to as PET polyester. In the crystalline PET polyester, a large drop in the molecular weight occurs when the polyester undergoes heat history in the course of molding processing, due to autohydrolysis caused by trace amounts of water contained in the polyester. At that time, the number of free carboxyl groups at a molecular terminal increases due to the cleavage of ester bonds, and a by-product of acetaldehyde by the thermal decomposition is practical problem, which has been a drawback to the development of recycling technologies for the collected bottles and sheets. The molecular weight of a used PET bottle that has been collected is reduced compared with brand-new pellets, for example, the molecular weight of the flakes (crushed products) of the collected PET bottles which are produced in a large amount is reduced to about one half. Accordingly, when the flakes of the collected PET bottles are recycled as a main material resin, the molding processability is poor, the flakes cannot be formed into the original PET bottle and can only be formed into filaments, sheets for an egg containers, and the like, which can be formed with the flakes having low molecular weight. Hence, the recycling applications of the flakes of collected PET bottles have been limited to a narrow range usage.
As methods to solve the above problems, there are known: a method of restoring the molecular weight by a solid phase polymerization of PET polyester; a method of reacting a chain extender with a hydroxyl group or a carboxylic group of a polyester terminal group; a method of adding another resin such as an elastomer to support mechanical properties; and the like.
As the chain extender, practical applications of compounds having functional groups such as isocyanate, oxazoline, epoxy, aziridine, and carbodiimide have been proposed. However, practical compounds are limited because of strict restrictions in reactivity, heat resistance, safety, stability, and the like.
Of these, an epoxy compound is relatively useful, and a blend of a monoepoxy compound (Patent Document 1), a blend of a diepoxy compound (Patent Document 2, Patent Document 3, and Patent Document 4), and the like have been disclosed. However, there have been various problems in reaction rate, by-production of gel, melt viscosity, compatibility, heat stability, physical properties of the molded product, and the like.
On the other hand, a method of increasing the molecular weight of polyester by mixing and melting the collected PET polyester with a bifunctional epoxy resin and a steric hindrance type hydroxyphenyl alkyl phosphonate has been proposed (Patent Document 5). This method may provide a relatively fast reaction rate, but the resin produced is in a linear structure, which is the same as the conventional PET, and hence, the increase in melt viscosity of the resin is small and the improvement effect of the molding processability is small. Further, the steric hindrance type hydroxyphenyl alkyl phosphonate to be used is expensive, and there is a problem with practical use in industry where low recovery and recycling costs are required. Further, there has also been proposed a method of blending a rubber or an elastomer with the polyester, but in such case, there have been difficulties in compatibility, heat resistance, elastic modulus, and the like.
The inventors of the present invention have previously proposed a method of adding a mixed epoxy resin of bifunctional epoxy resin and a trifunctional epoxy resin (low-molecular-weight liquid) and an organic acid metal salt catalyst to PET polyester and mixing and melting the mixture with a reactive extrusion method so that the resultant is formed into a long chain branched structure and the molecular weight of PET polyester is increased in a short period of time (Patent Document 6). After that, however, in the practical use of, in particular, a film, there has arisen a problem of by-production of gelling or fish eyes (FE) attributed to a heterogeneous local reaction in the course of a high-speed coupling reaction within several minutes.
Subsequently, a method of performing a homogeneous reaction using a masterbatch in which a mixed epoxy resin (low-molecular-weight liquid) of bifunctional epoxy resin and a trifunctional epoxy resin and an organic acid metal salt catalyst are diluted about 10-fold with PET polyester has been proposed (Patent Document 7). The method has improved to a large extent the by-production of gelling or fish eyes (FE) in the course of a high-speed coupling reaction within several minutes. In the masterbach method, however, it is difficult, in principle, to homogeneously mix a trace amount of the epoxy resin of low viscous liquid into the highly viscous resin inside a reactive extruder, and hence, there is needed an expensive reactive extruder having extremely specific structure for practical use. Further, in a long-term operation for several days or more, by-production of trace amounts of gel or fish eyes (FE) tends to increase with time due to the following reasons: degree of reaction completion is not uniform and hence the melt viscosity (MFR) of the pellets fluctuates; and a modifier is deposited in and adhered to the grooves of the screw.
Patent Document 1: JP 57-161124 A
Patent Document 2: JP 07-166419 A
Patent Document 3: JP 48-25074 B
Patent Document 4: JP 60-35944 B
Patent Document 5: JP 08-508776 A
Patent Document 6: JP 3503952 B
Patent Document 7: WO 2001/094443